1. Field of the Invention
The present invention relates to a high frequency cooking apparatus (microwave oven) that supplies electric power to a high frequency generator (magnetron) through a relay contact, sets a cycle configured with ON time and OFF time for the relay contact based on externally-set heating time and heating power set, and controls a power of a magnetron.
2. Description of Related Art
Recently, it is getting easy to control a commercial-use microwave oven just by selecting one from previously-set recipes (combination of heating time and heating power in accordance with a type and an amount of an object to be heated). The commercial-use microwave oven has high power of high frequency. For adjusting the heating power of the commercial-use microwave oven, a user adjusts time required for cyclically (e.g., 32 second cycle) turning on/off the high frequency oscillation of the magnetron.
FIG. 16 is a timing chart showing examples of ON/OFF time (second) for the power supply of magnetron and ON/OFF time (second) for high frequency oscillation of magnetron in accordance with the heating power (%).
For example, when the heating power is 90% in FIG. 16, the 32 second cycle is repeated in which ON time continues for 30 seconds and OFF time continues for 2 seconds. When the heating power is 60% in FIG. 16, the 32 second cycle is repeated in which ON time continues for 22 seconds and OFF time continues for 10 seconds. The turning on/off of the magnetron is implemented by the turning on/off a relay contact. The hatched portion in FIG. 16 represents oscillation rise time required for the magnetron, and the oscillation rise time does not contribute to heating. It should be noted that the heating power (%) represents a nominal value, and thus may not match to a calculated power.
The relay contact is degraded by the on/off action. Thus, the commercial-use microwave oven is configured to count the number of on/off actions, and to indicate the relay exchange timing when the counted number reaches to a predetermined value. The life of relay is considered in views of a mechanical point and an electrical point. The life in view of the electrical point is considered to correspond to the two hundred thousand times of on/off actions in the commercial-use microwave oven. Regardless of malfunctions, the relay should be exchanged when the counted number of on/off actions reaches to two hundred thousand, for the preventive maintenance.
Patent Document 1 discloses a microwave oven including: a high voltage transformer that actuates a magnetron; a relay that drives the high voltage transformer; a control circuit that outputs a relay drive signal for closing the relay; and a contact detection circuit that detects a condition in which a relay contact is closed. The microwave oven times an operation period from the output of relay drive signal to the closure of relay contact, and stores the operation time in an EEPROM. Then, at the time of starting an operation, the control circuit determines a phase that leads the minimum making current based on the operation time stored in the EEPROM, and outputs the relay drive signal with the determined phase for closing the relay contact.
Patent Document 1: Japanese Patent Application Laid-Open No. H05-205866